Automatic motor shut-off networks for signal seeking receivers



June 2, 1970 J. BUHR ,51

AUTOMATIC MOTOR SHUT-OFF NETWORKS FOR SIGNAL .SEEKiNG RECEIVERS FiledDec.. 19, 1966 I 15+ 1O 11 3 13 R7 R2 RB n 70 L1 D7 s1 52 R3 D2 12 F vv17 'C2 I I R4 W R1 fi TR2 TUSTEREOIAMP Too 1 INVENTOR. JACOB BUHR PATENTAGENT United States Patent 3,515,960 AUTOMATIC MOTOR SHUT-OFF NETWORKSFOR SIGNAL SEEKING RECEIVERS Jacob Buhr, Kitchener, Ontario, Canada,assignor to Electrohome Limited, Kitchener, Ontario, Canada Filed Dec.19, 1966, Ser. No. 602,941 Int. Cl. H04b 1/32 US. Cl. 318-16 8 ClaimsABSTRACT OF THE DISCLOSURE The motor which drives the tuning condenserof a signal seeking receiver is stopped automatically (a) in response tothe tuning of the receiver to a signal of sufficient strength and (b)after the motor has been operating for a predetermined period of timewithout the receiver being tuned to such a signal, the latter beingaccomplished by the charging of a capacitor while the motor is operatingand the automatic stopping of the motor when the capacitor charges to apredetermined voltage, this being followed by discharging of thecapacitor while the motor is stopped.

This invention relates to radio receivers of the signal seeking type.More particularly, this invention relates to networks for automaticallyturning off the motor of a signal seeking receiver (a) when the receiveris tuned to the frequency of a signalbeing received by the receiver andof a strength greater than a minimum predetermined signal strength, and(b) after the motor has been operating for a predetermined length oftime.

In any signal seeking receiver it is necessary to provide some means forturning on the motor that drives the tuning condenser of the receiverwhen the latter is turned on but no signal is being received, or when itis desired to change stations. These means must be capable ofautomatically shutting off the motor when the receiver is tuned to asignal of a level greater than a minimum predetermined level. It alsowould be desirable to provide means for shutting off the motorautomatically after it has been operating for some predetermined lengthof time. This will ensure that the motor will not run indefinitelyshould' any one ofthe following conditions be encountered: (a) signalsbeing received of insutficient signal strength; (b) antennadisconnected; or (0) top of sensitivity control set too low. Inaccordance with this invention, a network is provided for accomplishingthe foregoing objectives.

A signal seeking receiver embodying this invention is of a type havingvariable tuning means for varying the tuning of the receiver, a motordrivingly connected to the tuning means, whereby the tuning of thereceiver can be changed by operation of the motor, and an automaticshut-otf network for automatically turning off the motor when thereceiver is tuned to the frequency of a signal being received by thereceiver and of a strength greater than a minimum predetermined signalstrength.

In accordance with this invention, such a receiver is provided withmeans for automatically turning off the motor 'after the motor has beenoperating for a predetermined period of time without the receiver havingbeen tuned to one such received signal during the aforementioned period.

This invention will become more apparent from the following detaileddescription, taken in conjunction with the appended drawing which showsan automatic shut.- oif network embodying this invention.

Referring to the figure, there is shown a motor 12 whose drive shaft isconnected to the tuning capacitor 13 of a signal seeking receiver. Powerto operate motor 12 3,515,960 Patented June 2, 1970 is provided from anysuitable A.C. or DC. source, an A.C. source being schematicallyindicated in the figure as an A.C. generator 11, via the contacts S1(when closed) of a relay and a motor reversing network 10, which may beof a conventional type. In fact, motor reversing network 10 is notessential to the operation of this invention, since, when tuningcapacitor 13 reaches the limit of its travel in one direction, it couldbe returned manually to the other limit of its travel. The coil of therelay having contacts S1 is 'designated L1 in the figure.

The automatic shut-off network shown in the figure employs threetransistors: TR1, TR2 and TR4. A fourth transistor TR3 is employed forstereo only signal seeking. A terminal 15 is connected to the positiveterminal of a suitable D.C. source (B+), such as a battery or other DC.power supply. Terminal 15 is connected via coil L1 to the collectorelectrode of transistor TR2. The emitter electrode of this transistor isconected to a terminal 16 at a DC. reference potential, in the presentcase, ground potential. Terminal 15 also is connected via coil L1, adiode D2 and a resistor R3 to the base electrode of transistor TR1,whereby bias may be applied to the base electrode of transistor TR1. Acapacitor C1 is connected between the base electrode of transistor TR1and ground to prevent transistors TR1 and TR2 from oscillating via theloop consisting of diode D2 and resistor R3. The collector electrode oftransistor TR1 is connected via a resistor R4 to the base electrode oftransistor TR2. A resistor R5 is connected in voltage dividerrelationship with resistor R4 between terminal 16 and the base electrodeof transistor TR2. A capacitor C2 is connected between ground and thecollector electrode of transistor TR1 for the purpose of ensuring thatupon the application of B-lto terminal 15, transistor TR1, rather thantransistor TR2, will turn on. Terminal 15 also is connected to thecollector electrode of transistor TR1 via a resistor R2. A switch S3 isconnected between the base electrode of transistor TR1 and ground. Asignal input terminal 17 is connected to the base electrode oftransistor TR1 via an isolating diode D1, a resistor R1 and a diode D6.The signal applied to signal input terminal 17 may be derived, forexample from any one of the LP. amplifiers of the receiver, furtheramplified, if necessary, and then detected to provide a positive D.C.signal, the latter being applied to terminal 17. The emitter electrodeof transistor TR1 is connected to terminal 16.

A circuit consisting of a resistor 7, a diode D7 and switch S2 isconnected between terminal 15 and the base electrode of transistor T R3.A diode D5 having its anode connected to the anode of diode D6 isconnected between the emitter electrode of transistor TR3 and the commonterminal of resistor R1 and diode D6. A resistor R10 is connectedbetween the emitter electrode of transistor TR3 and ground. Thestereophonic indicator lamp 14 of the receiver is connected via aresistor R9 to ground and also via a diode D4 to the base electrode oftransistor TR3. Stereophonic indicator lamp 14 and resistor R9 are soconnected to the base electrode of transistor TR3 that when the receiveris tuned to an F.M. stereophonic signal, as opposed to an F.M. monauralsignal, there will be a sufficient voltage drop across resistor R9 toturn on transistor TR3, assuming that the received signal is ofsufficient strength, of course. Other arrangements to ensure the turn onof transistor TR3 with switch S2 open only when the receiver is tuned toa stereophonic signal having a strength greater than a minimumpredetermined strength could be used, of course.

The emitter electrode of transistor TR4 is connected to terminal 16,while its collector electrode is connected to the common terminal ofresistors R4 and R5. A time delay 3 network consisting of a resistor R8and a capacitor C10 isconnected between terminal and terminal 16, thecommon terminal of resistor R8 and capacitor C10 being connected to thebase electrode of transistor TR4 via diode D8 and to the collectorelectrode of transistor TR1 via diode D9.

The operation of the network just described now will be discussed. Whenrelay contacts S1 are closed, generator 11 will be connected to motor 12via closed contacts S1 and motor reversing network 10, and motor 12 willdrive tuning capacitor 13 to change the tuning of the receiver. Motorreversing network 10 can include microswitches that are operated whenthe tuning capacitor reaches the limits of its travel in bothdirections, operation of the microswitches causing motor 12 to reversedirection.

Transistors TR1 and TR2 are so connected that when one is turned on, theother is kept turned 01f, and vice versa until the state of conductionof the former transistor changes.

The collector and emitter electrodes of transistor TR2 are connected ina circuit through which current required in order for motor 12 tooperate must flow. In this respect, motor 12 only can operate providedthat transistor TR2 is turned on. Only under these conditions cansufiicient current pass from terminal 15 through coil L1 and thecollector and emitter electrodes of transistor TR2 to ground to closerelay contacts S1. Of course, rather than employing a relay, transistorTR2 could be connected in a circuit through which either the armature orthe field current of motor 12 must pass. Such an arrangement is shown,for example, in copending application Ser. No. 602,944 filed Dec. 19,1966, for Automatic Shut-01f Networks for Signal Seeking Receivers,Jacob Buhr.

With switch S3 open, switch S2 open or closed and a positive DC.potential, 13+, which may be 10 to 12 volts for example, applied toterminal 15, transistor TR1 will be biased on, even if there is no inputsignal present at signal input terminal 17. With transistor TR1 turnedon, transistor TR2 will be kept Off, and, since transistor TR2 must beturned on before motor 12 can start, motor 12 will not operate.

The path for the current required to turn on transistor TR1 includescoil L1, diode D2, resistor R3 and the baseemitter junction oftransistor TR1. Diode D2 provides a low impedance path for the turn oncurrent which assists in ensuring that transistor TR1 will be turned onbefore transistor TR2 when B+ is applied to terminal 15. It will beappreciated that if transistor TR1 were not turned on before transistorTR2, the latter transistor would be turned on due to current flowingfrom terminal 15 to terminal 16 via resistors R2, R4 and R5. This wouldresult in transistor TR1 losing its control function.

Diode D2 presents a high impedance to any positive signal appearing atthe base electrode of transistor TR1, by virtue of which excessiveloading of this signal is eliminated.

With switch S2 closed, the network will operate in such a manner thatmotor 12 will stop when the receiver is tuned to the frequency of asignal being received by the receiver and of a strength greater than aminimum predetermined signal strength regardless of whether the signalis a monaural signal or a stereophonic signal. With switch S2 closed,switch S3 open, and B+ applied to terminal 15, transistor TR1 will beturned on and will keep transistor TR2 turned off. Under thesecircumstances, the small current which will flow through coil L1 will beunable to close contacts S1, so motor 12 will not be operating. In orderto start motor 12, it is necessary to close momentarily switch S3, whichwill ground the base electrode of transistor TR1. When this is done, therelatively high voltage which, prior to the momentary closing of switchS3, had been applied to the base electrode of transistor .TRl via coilL1, diode D2 and resistor R3 and which kept this transistor turned on,immediately will decrease to ground potential causing transistor TR1 toturn ofi. When transistor TR1 turns off, the voltage at its collectorelectrode will rise, and this relatively high voltage will be applied tothe base electrode of transistor TR2 via the voltage divider networkconsisting of resistors R4 and R5, and, whereas when transistor TR1 wasturned on and its collector voltage was relatively low, thereby holdingtransistor TR2 oif, now transistor TR2 will turn on because of theincrease in the voltage which will be applied to its base electrode whentransistor TR1 is turned Off. The voltage at the collector electrode oftransistor TR2 will drop as soon as this transistor turns on, and thisrelatively low voltage will be applied to the base electrode oftransistor TR1 via diode D2 and resistor R3, thereby keeping transistorTR1 turned off. The voltage at the collector electrode of transistor TR2when it is turned on is dependent on the saturation voltage of thetransistor and typically may be of the order of +0.2 to +0.3 volt. Oncetransistor TR2 is turned on, a current suflicient to close contacts S1will flow from terminal 15 to terminal 16 via coil L1 and the collectorand emitter electrodes of transistor TR2 and motor 12 will begin tooperate.

It will be understood that when switch S2 is closed with switch S3 open,transistor TR3 also Will be turned on when B+ is applied to terminal 15.When transistor TR3 is turned on, a voltage will be developed acrossresistor R10 that will back bias diode D5, thereby preventing anyshunting effect on the base electrode of transistor TR1. With switch S2closed, transistor TR3 will remain turned on regardless of the state ofconduction of transistors TR1 and TR2.

Motor 12 will continue to run for a predetermined length of time oruntil the receiver becomes tuned to either a monaural or a stereophonicsignal of a strength greater than a minimum predetermined strength,which ever first occurs in time. When the receiver is tuned to such asignal, an input signal will be applied to input terminal 17. Thissignal will appear as a positive D.C. voltage at the base electrode oftransistor TR1. Provided that the signal at the base electrode oftransistor TR1 is above a minimum level, transistor TR1 will turn on.Transistor TR2 then will be turned off, by virtue of which the currentflowing through coil L1 will be decreased to a value insufficient tohold contacts S1 closed, as a result of which motor 12 will stop.

In order to tune another station, it is only necessary to momentarilyclose switch S3 again, whereupon the sequence of events outlinedhereinbefore will be repeated.

However, as soon as transistor TR1 is turned off, thereby causingtransistor TR2 to turn on and motor 12 to operate, capacitor C10 willbegin to charge from B+ via resistor R8. When the voltage acrosscapacitor C10 is suflicient to exceed the forward voltage drop of diodeD8 and the forward base-emitter voltage drop of transistor TR4, it willcause transistor TR4 to turn on. Naturally resistor R8 and capacitor C10will have to be chosen so that sufiicient current can flow throughresistor R8 to turn on transistor TR4. When transistor TR4 is turned on,its collector voltage will drop to a value less than the V of transistorTR2 (the saturation voltage drop of transistor TR4 is less than the V oftransistor TR2), and transistor TR2 will turn off, which will result inthe stopping of motor 12 and the turn on of transistor TR1. CapacitorC10 then will discharge via diode D9 and the collectoremitter circuit oftransistor TR1 to a level determined by the forward voltage drop ofdiode D9 and the saturation voltage drop of transistor TR1. Since thisvoltage will be lower than that required to keep transistor TR4 turnedon (forward voltage drop of diode D9 plus saturation voltage oftransistor TR1 less than the forward voltage drop of diode D8 plus V oftransistor TR4), transistor TR4 will turn oif, and it only can be turnedon again by the charging up of capacitor C10.

In order to cause motor 12 to operate again, switch S3 should be closed.This will cause the voltage at the collector electrode of transistor TR1to rise,which, in turn will back bias diode D9, thereby permittingcapacitor C10 to charge up again to a voltage level where transistor TR4will conduct and turn off transistor TR2, assuming, of course, thatmotor 12 has not been stopped previously by virtue of the receiver beingtuned to a signal being received by the receiver and of a strengthgreater than a minimum predetermined strength. In the event that thelatter should occur, capacitor C10 will discharge when transistor TR1turns on in response to tuning in of a signal.

After the initial cycle described hereinbefore, capacitor C10 willcommence charging from a low voltage determined by the saturationvoltage drop of transistor TR1 and the forward voltage drop of diode D9to a relatively higher voltage equal to the forward voltage drop acrossdiode D8 and the forward base-emitter voltage drop of transistor TR4.The time taken for capacitor C10 to charge from the low voltage to thehigh voltage is determined by the values of resistor R8 and capacitorC10, the number of diodes in the base circuit of transistor TR4, thesaturation voltage drop of transistor TR1 and the forward voltage dropof diode D9. If additional diodes are added in series with diode D8 inthe base circuit of transistor TR4, the time required for capacitor C10to charge to a voltage suflicient to turn on transistor TR4 can beincreased.

It will be seen from the foregoing that if during the interval requiredfor capacitor C10 to charge to a voltage sufficient to turn ontransistor TR4, motor 12 has not been stopped automatically by virtue ofthe application to terminal 17 of a signal indicative of the receiverbeing tuned to the frequency of a signal being broadcast by a radiostation and of a level greater than a minimum predetermined level,transistor TR2 will be turned off automatically and motor 12 will stopoperating after capacitor C10 has charged to a value sufficient to turnon transistor TR4, i.e. after a predetermined period has elapsed.

If it is desired to listen only to stations broadcasting stereophonicsignals, switch S2 should be opened. In the absence of the receiverbeing tuned to a stereophonic signal, transistor TR3 will not normallyconduct, and any signal indicative of reception of a monaural signal andapplied to the base electrode of transistor TR1 via diode D1 andresistor R1 will be shunted to ground via diode D5 and resistor R10, sothat transistor TR1 will not turn on if resistors R1 and R18 areproperly chosen. The voltage at the junction of diodes D5 and D6 willhave to be of the order of +0.8 volt in order to turn on transistor TR1.Since the forward voltage drop of diode D5 will be of the order of +0.6volt, the drop across resistor R10 must exceed about +0.2 volt beforetransistorv TR1 will turn on. By choosing resistors R1 and R10 relativeto the maximum voltage which will appear at input terminal 17,transistor TR1 can be prevented from turning on for any monaural signal.

When the receiver is tuned to a stereophonic signal, a voltage will bedeveloped across resistor R9. This voltage will be applied via diode D4to the base electrode of transistor TR3. The voltage applied to the baseelectrode of transistor TR3 via diode D4 will turn on transistor TR3,assuming the signal to be of sufiicient strength, and a voltage thenwill be developed across resistor R10 which will be approximately 0.6volt (forward base-emitter drop of transistor TR3) below the basevoltage of transistor TR3. The voltage developed across resistor R10under these circumstances will back bias diode D5, so that transistorTR1 then can be turned on by the signal applied to input terminal 17.When transistor TR1 is turned on, transistor TR2 will be turned off andmotor 12 will stop. The relatively high voltage appearing at thecollector electrode of transistor TR2 when it is turned off will keeptransistor TR1 turned on via diode D2 and resistor R3.

In order to select another station broadcasting a stereophonic program,it is only necessary to momentarily close switch S3 again. This willcause transistor TR1 to turn off, which, in turn will cause transistorTR2 to turn on 6 and motor 12 to operate. Transistor TR3 will turn offwhen the tuner is pulled off station by operation of motor 12, and itwill not normally turn on again until the re ceiver is tuned to anotherstereophonic signal.

However, if during the interval required for capacitor C10 to charge toa voltage sufficient to turn on transistor TR4, motor 12 has not beenstopped by virtue of the tuning of the receiver to a stereophonicsignal, transistor TR4 will be turned on and will turn off transistorTR2. The resulting increase in the collector voltage of transistor TR2will cause transistor TR1 to turn on, providing a path for discharge ofcapacitor C10, and the circuit will function in the same manner ashereinbefore discussed.

It should be appreciated that by adding other diodes at the junction ofdiode D1 and resistor R1, other control voltages may be used, e.g.,control voltages indicative of tuning to an AM. station. Since theseadditional diodes will be reverse bias connected with respect to diodeD1 and each other, no appreciable loading will take place.

It should be noted that diode D4 serves to prevent current flowingthrough resistor R7 and diode D7 from passing through indicator lamp 14when switch S2 is closed. It also will prevent noise pulses having amagnitude less than the diode voltage of diode D4 from turning ontransistor TR3 when switch S2 is open.

Capacitor C2 ensures that the voltage applied to the base electrode oftransistor TR2 when B+ is applied initially to terminal 15 will be keptrelatively low. This aids in ensuring that transistor TR1 will be turnedon before transistor TR2 when B+ is applied initially to terminal 15.

As aforementioned, the function of capacitor C1 is to preventoscillation of the trigger circuit consisting of transistors TR1 and TR2and the components which interconnect the same.

It should be appreciated that transistor TR3 and its associatedcomponents are not essential to the practice of this invention. Thistransistor and its associated components are employed for the purpose ofproviding an automatic shut-off network which is selective with respectto stereophonic signals. The instant invention is applicable regardlessof the type of signal being received by the receiver and regardless ofwhether the receiver will lock in only in respect of one type of signalor in respect of all types of signals.

The stereo only signal seeking feature of the network described hereinis described and claimed in copending application Ser. No. 602,940,filed Dec. 19, 1966, for Selective Automatic Motor Shut-Off Networks forSignal Seeking Receivers, Jacob Buhr.

While it is important that a path be provided that permits capacitor C10to discharge to a voltage lower than that required to cause transistorTR4 to turn on, and that this path be established in response to theturn on of transistor TR1, it is not necessary for transistor TR1 toconstitute a part of this path. Thus, for example, the discharge pathcould include another transistor not shown in the figure but connectedto turn on and off simultaneously with the turn on and off of transistorTR1.

While preferred embodiments of this invention have been disclosedherein, those skilled in the art will appreciate that changes andmodifications may be made therein without departing from the spirit andscope of this invention as defined in the appended claims.

What I claim as my invention is:

1. A signal seeking receiver comprising variable tuning means forvarying the tuning of said receiver; a

motor drivingly connected to said tuning means, whereby the tuning ofsaid receiver can be changed by operation of said motor; an automaticshut-off network for automatically turning off said motor when saidreceiver is tuned to the frequency of a signal being received by saidreceiver and of a strength greater than a minimum predetermined signalstrength, said automatic shut-off network comprising first and secondtransistors each having base, collector and emitter electrodes, saidtransistors being interconnected such that when either of saidtransistors is turned on or off, the other of said transistors is keptturn off or on respectively until the state of conduction of the firstmentioned transistor changes; said signal seeking receiver alsoincluding means for supplying a first signal to said base electrode ofsaid first transistor when said receiver is tuned to the frequency of asignal being received by said receiver to turn on said first transistorwhen said first signal is of a strength greater than a minimumpredetermined signal strength; means connecting said collector andemitter electrodes of said second transistor in a circuit through whichcurrent required in order for said motor to operate must pass, wherebywhen said seond transistor is turned ofi, said current is unable to flowthrough said circuit and said motor ceases operating; means forsupplying a biasing voltage to said first transistor to turn on saidfirst transistor when said motor is not operating; switching meanshaving first and second difierent states and which initiate operation ofsaid motor when in said second state; and means responsive to saidswitching means being in said second state providing a path for reducingsaid biasing voltage supplied to said first transistor below thatrequired to keep said first transistor turned on, whereby said firsttransistor is turned off when said switching means is in said secondstate; and means for automatically turning off said motor after saidmotor has been operating for a predetermined period of time without saidreceiver having been tuned to one of said received signals during saidperiod, said means for automatically turning 011 said motor after saidmotor has been operating for said predetermined period of timecomprising means for automatically turning off said second transistorafter said motor has been operating for said predetermined period oftime without said receiver having been tuned to one of said receivedsignals during said period.

2. The invention according to claim 1 wherein the last-mentioned meansinclude a third transistor, and means for turningsaid third transistoron after said predetermined period, said second and third transistorsbeing interconnected such that when said third transistor is turned on,said second transistor will be turned off.

3. The invention according to claim 1 wherein the last-mentioned meanscomprise a third transistor having base, collector and emitterelectrodes, a first diode, a first resistor and a first capacitor, saidsecond and third transistors being inter-connected such that when saidthird transistor is turned on, said second transistor will be turnedoil, said first capacitor being connected to be charged via said firstresistor, said first diode being connected between said first capacitorand said base electrode of said third transistor and providing a pathfor turn on of said third transistor after said first capacitor hascharged to a first voltage greater than the forward voltage drop of saidfirst diode plus the forward base-emitter voltage drop of said thirdtransistor, and a network providing a path for discharge of said firstcapacitor to a second voltage less than said first voltage in responseto said first transistor being turned on.

4. The invention according to claim 1 wherein the last-mentioned meanscomprise a third transistor having base, collector and emitterelectrodes, first and second diodes, a first resistor, and a firstcapacitor, said second and third transistors being interconnected suchthat when said third transistor is turned on, said second transistorwill be turned ofl", said collector and emitter electrodes of said thirdtransistor being connected between said base and emitter electrode ofsaid second transistor, said first capacitor being connected to becharged via said first resistor, said first diode being connectedbetween said first capacitor and said base electrode of said thirdtransistor and providing a path for turn on of said third transistorafter said first capacitor has charged to a first voltage greater thanthe forward voltage drop of said first diode plus the forwardbase-emitter voltage drop of said third transistor, and circuit meansproviding a path for discharge of said first capacitor to a secondvoltage lower than said first voltage when said first transistor isturned on, said circuit means including said second diode and saidcollector and emitter electrodes of said first transistor, said seconddiode being connected between said first capacitor and said collectorelectrode of said first transistor and being reverse biased when saidfirst transistor in turned off.

5. The invention according to claim 4 including a second resistorconnected between said collector electrode of said first transistor andsaid base electrode of said second transistor, a third resistorconnected in voltage divider relationship with said second resistor andconnected between said base electrode of said second transistor and aterminal at a reference potential, means connecting said emitterelectrodes of said first and second transistors and said terminal, and athird diode and a fourth resistor connected in series circuit with eachother between said collector electrode of said second transistor andsaid base electrode of said first transistor.

6. The invention according to claim 5 wherein said means for supplying abiasing voltage to said first transistor comprises a DC. power supply,said third diode and said fourth resistor, said DC. power supply alsobeing connected to said collector electrode of said second transistorand to said collector electrode of said first transistor and being thesource of charging current for said first capacitor, said first resistorbeing connected between said D.C. power supply and said first capacitor.

7. In a signal seeking receiver of a type having variable tuning meansfor varying the tuning of said receiver, a motor drivingly connected tosaid tuning means, whereby the tuning of said receiver can be changed byoperation of said motor, and an automatic shut-off network forautomatically turning off said motor when said receiver is tuned to thefrequency of a signal being received by said receiver and of a strengthgreater than a minimum predetermined signal strength, said automaticshut-01f network including a circuit through which current required inorder for said motor to operate must pass and switching means foropening and closing said circuit; means for automatically turning ofisaid motor after said motor has been operating for a predeterminedperiod of time without said receiver having been tuned to one of saidreceived signals during said period including a capacitor, means forcharging said capacitor While said motor is operating, means responsiveto said capacitor charging to a predetermined voltage while said motoris operating for supplying a signal to said switching means to open saidcircuit, and means providing a path for discharge of said capacitor whensaid motor is stopped to a voltage a less than the voltage to which saidcapacitor charges while said motor is operating.

8. The invention according to claim 1 wherein said switching means is atransistor.

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